If your buddy was using a powder appropriate for the 5.56,like,any powder in the loading manual,and if that powder was not contaminated with handgun powder in the powder measure or something...

The 5.56/.223 does not have enough case capacity to put enough powder in to blow up the gun.

You can overload,open up the primer pocket,blow the primer,but that won't make the gun come unglued to the degree you experienced.

What may be more likely,bang,bang,bangclick,no bang! Hmmmm,must not have fed a round (wrong) Pull back the op rod,try again..KABOOM!

If you do not put powder in a round,the primer drives the bullet into the barrel,but it does not kick or make noise.When you put the next round in,the barrel is plugged by a bullet.That will make your gun come unglued.

I have heard of uncrimped bullets,or insufficient neck tension bullets,being driven back into the case during feeding causing extra high pressure,and maybe a blowup,but I'm not too sure about that.

Lesson!!Unless someone is really shooting at you,if the gun does not go bang when it should go bang,(handguns,too)STOP!

Clear the gun,MAKE SURE THE CHAMBER IS EMPTY!

Push the takedown pin,hinge it open,pull the bcg,and see if the hole goes all the way through the barrel.(Looking in the muzzle has you standing there with the gun pointed at your head.Bad form.If you open it up and look through from the breech,you look like you know what you are doing)

And,yeah,the idea of refitting a barrel extension is bad.

Assembling an upper requires some tools,some skills,and an understanding of what you are doing.Pick an upper assembly,get it on order,and be patient.